Prosthetic disc for intervertebral insertion

ABSTRACT

A prosthetic disc for insertion between adjacent vertebrae includes a core having upper and lower curved surfaces, upper and lower plates, and peripheral restraining structure on at least one of the upper plate, the lower plate and the core. Each plate has an outer surface which engages a vertebra and an inner curved surface which slides over the curved surface of the core. The peripheral restraining structure serves to hold the core against a curved surface of at least one of the plates during sliding movement of the plates over the core.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 16/247,381, filed Jan. 14, 2019, now abandoned, which is acontinuation of U.S. patent application Ser. No. 15/916,139, filed Mar.8, 2018, now U.S. Pat. No. 10,219,911, which is a continuation of U.S.patent application Ser. No. 14/612,431, filed Feb. 3, 2015, now U.S.Pat. No. 10,052,211, which is a continuation of U.S. patent applicationSer. No. 14/150,437, filed Jan. 8, 2014, now U.S. Pat. No. 8,974,533,which is a continuation of U.S. patent application Ser. No. 12/626,027,filed Nov. 25, 2009, now U.S. Pat. No. 8,845,729, which is acontinuation of U.S. patent application Ser. No. 10/855,253, filed May26, 2004, now U.S. Pat. No. 7,753,956, which claims the benefit of U.S.Provisional No. 60/473,802, filed May 27, 2003, and U.S. Provisional No.60/473,803, filed May 27, 2003; all of which are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to medical devices and methods. Morespecifically, the invention relates to a prosthetic disc forintervertebral insertion, such as in the lumbar and cervical spine.

In the event of damage to a lumbar or cervical intervertebral disc, onepossible surgical treatment is to replace the damaged disc with a discprosthesis. Several types of intervertebral disc prostheses arecurrently available. For example, one type of intervertebral discprosthesis is provided by Waldemar Link GmbH & Co under the trademarkLINK SB CHARITE™. This prosthesis includes upper and lower prosthesisplates or shells which locate against and engage the adjacent vertebralbodies, and a low friction core between the plates. The core has upperand lower convexly curved surfaces and the plates have corresponding,concavely curved recesses which cooperate with the curved surfaces ofthe core. This allows the plates to slide over the core to allowrequired spinal movements to take place. The curved recesses in theplates are surrounded by annular ridges which locate, at the limit ofsliding movement of the plates over the core, in opposing upwardly anddownwardly facing, peripheral channels surrounding the curved surfacesof the core.

This type of disc configuration is described in EP 1142544A1 and EP1250898A1, assigned to Waldemar Link GmbH & Co. A drawback of suchconfigurations is that the provision of the peripheral ribs and channelslimits the areas available for bearing and sliding contact between theplates and core, and accordingly the loads which can be transmitted bythe prosthesis. As a result of the relatively small bearing areas, it isbelieved that at least the core will be subject to rapid wear and have arelatively short lifespan. Also, because the core is in effect merely“clamped” between the plates, this configuration does not allow forsecure retention of the core. In one alternative arrangement, the curvedsurfaces of the core carry opposing, elongate keys that locate inelongate grooves in the plates and another alternative arrangement inwhich the plates have opposing elongate keys that locate in elongategrooves in the opposite curved surfaces of the core. These key andgroove arrangements allow the plates to slide over the core within thelimits of the length of the grooves, in one direction only. Althoughallowance is made for some lateral play of the keys in the grooves, verylittle sliding movement of the plates over the core can take place inthe orthogonal vertical plane, and this is considered to be a seriousdrawback of this design.

Other currently available intervertebral disc prostheses have similarand/or other drawbacks. Typically, drawbacks include insufficientresistance to wear and tear, restricted range of motion and/orinsufficient ability of the prosthesis to adhere to vertebral bone.

Therefore, a need exists for improved intervertebral disc prostheses.Ideally, such improved prostheses would resist wear and tear, provide adesired range of motion and adhere well to vertebral bone. At least someof these objectives will be met by the present invention.

2. Description of the Background Art

Published US patent applications 2002/0035400A1 and 2002/0128715A1describe disc implants which comprise opposing plates with a corebetween them over which the plates can slide. The core receives one ormore central posts, which are carried by the plates and which locate inopposite ends of a central opening in the core. Such arrangements limitthe load bearing area available between the plates and core.

Other patents related to intervertebral disc prostheses include U.S.Pat. Nos. 4,759,766; 4,863,477; 4,997,432; 5,035,716; 5,071,437;5,370,697; 5,401,269; 5,507,816; 5,534,030; 5,556,431; 5,674,296;5,676,702; 5,702,450; 5,824,094; 5,865,846; 5,989,291; 6,001,130;6,022,376; 6,039,763; 6,139,579; 6,156,067; 6,162,252; 6,315,797;6,348,071; 6,368,350; 6,416,551; 6,592,624; 6,607,558 and 6,706,068.Other patent applications related to intervertebral disc prosthesesinclude U.S. Patent Application Publication Nos.: 2003/0009224;2003/0074076; 2003/0191536; 2003/0208271; 2003/0135277; 2003/0199982;2001/0016773 and 2003/0100951. Other related patents include WO01/01893A1, EP 1344507, EP 1344506, EP 1250898, EP 1306064, EP 1344508,EP 1344493, EP 1417940, EP 1142544, and EP 0333990.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the present invention, a prosthetic disc for insertionbetween adjacent vertebrae includes upper and lower plates having outersurfaces, which engage and are locatable against the respectivevertebrae, and inner curved surfaces. A core is disposed between thecurved surfaces to allow the plates to slide over the core. Preferably,the plates can slide freely in all directions, not being limited tomovement in a single direction as with the prior art. The presentinvention further provides peripheral restraining structure on one orboth of the plates or the core to hold the core against the curvedsurface of at least one of the plates during sliding movement of theplates over the core. The peripheral restraining structure defines alimit or boundary for movement of the core relative to at least one ofthe upper and lower plates. Within such a peripheral boundary, however,movement of the core relative to the plate will preferably beunconstrained. That is, movement of the core relative to the plate mayoccur in any direction without significant inhibition or friction. Thecore will preferably not be attached to either the upper or lower plate,and the plates will thus be able to freely articulate relative to eachother over the core, which provides a low friction bearing surface.

An advantage of the structure thus described is that the surface contactarea between the core and each of the upper and lower plates may bemaximized. By providing only a peripheral restraint, as opposed forexample to grooves and keys on the surface of the core and plates, thewidth or diameter of the core relative to the size of the plate may bemaximized. Moreover, the surfaces of the core and the plates whichcontact each other may be made smooth and free from other structure(s)that might adversely affect performance. In the preferred embodiments,both the curved surfaces of the plates and the corresponding surfaces ofthe core will be spherical sections. The use of spherical surfacespromotes free, unconstrained relative motion of the plates and the corein all directions.

In some embodiments, the peripheral restraining structure limitsrelative inclination of the plates during sliding movement of the platesover the core, usually by defining a stop structure. In otherembodiments, the peripheral restraining structure lifts one side of thecore relative to an opposite side of the core during sliding movement ofthe plates over the core. The peripheral restraining structure itselfmay take any of a number of different forms. In one embodiment, forexample, the restraining structure comprises a ring structure on atleast one of the upper and lower plates and an annular structure on atleast a portion of the periphery of the core. The ring structure will beadapted to engage and restrain the annular structure on the core. Forexample, the ring structure may comprise a flange which defines anoverhang over at least a portion of the periphery of one of the plates.The overhang of the flange will receive the annular structure on thecore to provide an interference fit which retains the core against thecurved surface of the plate but allows the core to slide freely and inan unconstrained manner within the limit or boundary defined by theflange. The annular structure on the core may be a rim which extendscontinuously or discontinuously (preferably continuously) around alateral circumference of the core. By providing a rim which has a width,usually a diameter, which is slightly greater than the correspondingwidth of an inner edge of the flange at one point, the core will be heldin place and will not be dislodged from the cavity defined by the ringstructure in normal use.

Usually, the flange or other ring structure as well as the rim or otherannular structure will be formed continuously about the periphery of theplate and core, respectively. Alternatively, however, either or both ofthe annular structure and the ring structure could be formeddiscontinuously. That is, so long as at least some portion of the ringstructure and the annular structure remain engaged during all expectedgeometries and uses of the prosthetic disc, the objective of holding thecore against the curved surface of the plate will be met.

The upper and lower plates may be made of any suitable material orcombination of materials, such as but not limited to cobalt chromemolybdenum and titanium. In some embodiments, titanium plates are used,and these plates may optionally include inner surfaces of titaniumnitride and outer surfaces that are aluminum oxide blasted to createmicro-concavities. In another embodiment, cobalt chrome plates are used,with the outer surfaces being blasted with aluminum oxide and thencoated with a titanium plasma spray. In some embodiments, the platescomprise an MRI-compatible material, such as titanium, coupled with ahardened material, such as cobalt chrome molybdenum. Such materials maybe coupled using any suitable means, such as laminating, slip fitting,interferences fitting, adhesion, welding, molding or the like. Someplates include a coating or material on the inner surfaces for reducingfriction and/or wear and tear, such as a titanium nitride surface.

Optionally, in some embodiments the outer surfaces of the upper andlower plates have at least one surface feature for promoting attachmentof the outer surfaces to the vertebrae. For example, such surfacefeatures may include a plurality of serrations disposed along the outersurfaces. Some embodiments include additional or alternative features onthe outer surfaces for enhancing attachment of the prosthesis tovertebral bone, such as a material or coating, like a titanium plasmaspray. Multiple micro-concavities may be formed on the outer surfaces,for example by aluminum oxide spraying, to further enhance attachment.Additionally or alternatively, the surface features may include at leastone fin disposed on each of the outer surfaces. In some embodiments, thefin includes at least one hole for further promoting attachment to thevertebrae. Fins may extend vertically from their corresponding outersurfaces at right angles, or alternatively the fins may extend fromtheir corresponding outer surface at angles other than 90°. Fins mayalso have any suitable orientation relative to the anterior-posterioraxis of the prosthesis. For example, a fin may extend in a straight linefrom anterior to posterior, without being angled. Alternatively, the finmay be rotated or angled away from the anterior-posterior axis at anysuitable angle between 0° and 180°. In one embodiment, each fin isdisposed in a lateral orientation on the outer surfaces.

The core may generally have any suitable configuration and be made ofany suitable material or combination of materials, such as polymers,ceramics or the like. In some embodiments, the core comprises alow-friction material and has two convex surfaces for slidably engagingthe inner, curved surfaces of the upper and lower plates.

In another aspect of the present invention, a prosthetic disc forinsertion between adjacent vertebrae includes upper and lower plates anda free-floating core disposed between the plates. Again, the upper andlower plates have outer surfaces locatable against the respectivevertebrae and inner, curved surfaces. Additionally, at least one of theupper and lower plates includes a flange extending from one of the innersurfaces. The core includes at least one peripheral groove for engagingwith the flange(s) to hold the core captive between the plates duringsliding movement of the plates over the core. Any of the featuresdescribed above may also be incorporated in various embodiments.

In another aspect of the present invention, a prosthetic disc forinsertion between adjacent vertebrae includes upper and lower plateshaving outer surfaces locatable against the respective vertebrae andinner, curved surfaces, at least one of the upper and lower platesincluding a flange extending from one of the inner surfaces. Afree-floating core is disposed between the curved surfaces to allow theplates to slide over the core, and the core includes at least oneperipheral protrusion for engaging with the flange(s) to hold the corecaptive between the plates during sliding movement of the plates overthe core. Again, various embodiments may include any of the featuresdescribed above.

In yet another aspect of the invention, a prosthetic disc for insertionbetween adjacent vertebrae includes upper and lower plates having outersurfaces locatable against the respective vertebrae and inner curvedsurfaces, a core between the plates, and opposing retaining formations.The core includes upper and lower curved surfaces complementary in shapeto the inner, curved surfaces of the plates to allow the plates to slideover the core, the upper and lower surfaces of the core being locatedrespectively above and below an equatorial plane extending laterallythrough the core. The opposing retaining formations are locatedperipherally on the equatorial plane of the core and at an edge of thecurved surface of at least one of the plates and serve to hold the corecaptive between the plates during sliding movement of the plates overthe core.

In yet another aspect of the invention, a method for restraining spacingbetween adjacent vertebrae involves implanting an upper plate against alower surface of an upper vertebral body, implanting a lower plateagainst an upper surface of a lower vertebral body, and disposing a corebetween the upper and lower plates The core floats between sphericalcavities in each of the upper and lower plates, the plates restrainingperipheral movement of the core using at least one peripheralrestraining member. In some embodiments, implanting each of the platescomprises sliding a fin on each plate into a corresponding groove formedin its respective vertebral body. The fin may slide into the groove inany suitable direction, such as posterior-anterior, anterior-posterior,lateral, or any angled direction between an anterior-posteriororientation and a lateral orientation. Optionally, implanting mayfurther involve contacting textured outer surfaces of the upper andlower plates with the upper and lower surfaces of the vertebral bodies.

In another aspect of the invention, a method for assembling a prostheticdisc for insertion between adjacent vertebrae involves movably couplinga core with a first endplate to form an interference fit between thecore and the first endplate and contacting the core with a secondendplate. In some embodiments, coupling the core with the first endplatecomprises snap fitting the core into the endplate. Alternatively,coupling the core with the first endplate may comprise forming theendplate around the core. In some embodiments, coupling the core withthe first endplate involves engaging a peripheral protrusion of the corewith a peripheral restraining structure of the first endplate.

These and other aspects and embodiments will be described in furtherdetail below, with reference to the drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional anterior view of a prosthetic disc withthe prosthesis plates and core in vertical alignment, according to oneembodiment of the present invention;

FIG. 2 shows a side view of the prosthetic disc in FIG. 1 after slidingmovement of the plates over the core;

FIG. 3 shows a side view of the prosthetic disc in FIG. 1 aftertranslational movement of the plates relative to the core;

FIG. 4 shows a side view of the prosthetic disc in FIG. 1 with theprosthesis plates and core in vertical alignment;

FIG. 5 shows a plan view of a core of a prosthetic disc, according toone embodiment of the present invention; and

FIG. 6 shows a plan view of an upper plate of a prosthetic disc,according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 to 4 illustrate a prosthetic disc 10 for intervertebralinsertion between two adjacent spinal vertebrae (not shown). The disc 10comprises three components, namely an upper plate or shell 12, a lowerplate or shell 14 and a core 16 located between the plates.

The upper plate 12 includes an outer surface 18 and an inner surface 24and may be constructed from any suitable material or combination ofmaterials, such as but not limited to cobalt chrome molybdenum, titanium(such as grade 5 titanium) and/or the like. In one embodiment, typicallyused in the lumbar spine, the upper plate 12 is constructed of cobaltchrome molybdenum, and the outer surface 18 is treated with aluminumoxide blasting followed by a titanium plasma spray. In anotherembodiment, typically used in the cervical spine, the upper plate 12 isconstructed of titanium, the inner surface 24 is coated with titaniumnitride, and the outer surface 18 is treated with aluminum oxideblasting. An alternative cervical spine embodiment includes no coatingon the inner surface 24. In some embodiments, it may be useful to coupletwo materials together to form the inner surface 24 and the outersurface 18. For example, the upper plate 12 may be made of anMRI-compatible material, such as titanium, but may include a hardermaterial, such as cobalt chrome molybdenum, for the inner surface 24.Any suitable technique may be used to couple materials together, such assnap fitting, slip fitting, lamination, interference fitting, use ofadhesives, welding and/or the like. Any other suitable combination ofmaterials and coatings may be employed in various embodiments of theinvention.

In some embodiments, the outer surface 18 is planar. Oftentimes, theouter surface 18 will include one or more surface features and/ormaterials to enhance attachment of the prosthesis 10 to vertebral bone.For example, the outer surface 18 may be machined to have a serrations20 or other surface features for promoting adhesion of the upper plate12 to a vertebra. In the embodiment shown (FIG. 6 ), the serrations 20extend in mutually orthogonal directions, but other geometries wouldalso be useful. Additionally, the outer surface 18 may be provided witha rough microfinish formed by blasting with aluminum oxidemicroparticles or the like. In some embodiments, the outer surface mayalso be titanium plasma sprayed to further enhance attachment of theouter surface 18 to vertebral bone.

The outer surface 18 may also carry an upstanding, vertical fin 22extending in an anterior-posterior direction. The fin 22 is pierced bytransverse holes 23. In alternative embodiments, the fin 22 may berotated away from the anterior-posterior axis, such as in alateral-lateral orientation, a posterolateral-anterolateral orientation,or the like. In some embodiments, the fin 22 may extend from the surface18 at an angle other than 90°. Furthermore, multiple fins 22 may beattached to the surface 18 and/or the fin 22 may have any other suitableconfiguration, in various embodiments. In other embodiments, the fin 22In some embodiments, such as discs 10 for cervical insertion, the fins22, 42 may be omitted altogether.

The inner, spherically curved concave surface 24 is formed at a central,axial position with a circular recess 26 as illustrated. At the outeredge of the curved surface 24, the upper plate 12 carries peripheralrestraining structure comprising an integral ring structure 26 includingan inwardly directed rib or flange 28. The flange 28 forms part of aU-shaped member 30 joined to the major part of the plate by an annularweb 32. The flange 28 has an inwardly tapering shape and defines upperand lower surfaces 34 and 36 respectively which are inclined slightlyrelative to the horizontal when the upper plate 12 is at the orientationseen in FIG. 1 . An overhang 38 of the U-shaped member 30 has a verticaldimension that tapers inwardly as illustrated.

The lower plate 14 is similar to the upper plate 12 except for theabsence of the peripheral restraining structure 26. Thus, the lowerplate 14 has an outer surface 40 which is planar, serrated andmicrofinished like the outer surface 18 of the upper plate 12. The lowerplate 14 optionally carries a fin 42 similar to the fin 22 of the upperplate. The inner surface 44 of the lower plate 14 is concavely,spherically curved with a radius of curvature matching that of the innersurface 24 of the upper plate 12. Once again, this surface may beprovided with a titanium nitride or other finish.

At the outer edge of the inner curved surface 44, the lower plate 14 isprovided with an inclined ledge formation 46. Alternatively, the lowerplate 14 may include peripheral restraining structure analogous to theperipheral restraining structure 26 on the upper plate 12.

The core 16 of the disc 10 is made of a low-friction material, such aspolyethylene (Chirulen™). In alternative embodiments, the core 16 maycomprise any other suitable material, such as other polymers, ceramicsor the like. The core 16 has identical upper and lower sphericallycurved convex surfaces 48, 50. The radius of curvature of these surfacesmatches the radius of curvature of the inner surfaces 24, 44 of theupper and lower plates 12, 14. The curved surfaces are accordinglycomplementary. For wear resistance, the surface zones of the core may behardened by an appropriate cross-linking procedure.

The core 16 is symmetrical about a central, equatorial plane 52 whichbisects it laterally. (Although in other embodiments, the core 16 may beasymmetrical.) Lying on this equatorial plane is an annular recess orgroove 54 which extends about the periphery of the core. The groove 54is defined between upper and lower ribs or lips 56. When the plates 12,14 and core 16 are assembled and in the orientation seen in FIG. 1 , theflange 28 lies on the equatorial plane and directly aligned with thegroove 54. The outer diameter 58 of the lips 56 is preferably veryslightly larger than the diameter 60 defined by the inner edge of theflange 28. Assembly of the core and upper plate may involve pressing thecore through the circular aperture defined by the flange 28, with theinherent resilience of the core allowing the minor deformation of theupper rib 56, or that the core be introduced at an inclination. In otherless preferred embodiments of the invention (not shown), the diameter 58may be equal to or even slightly less than the diameter 60.

In some embodiments, the inner surface of the groove 54 may be provided,for wear resistance, with a lining of pure titanium or titaniumimpregnated with cobalt chrome, titanium nitride, other titanium alloyor the like.

The central axis of the disc 10 (the axis passing through the centers ofcurvature of the curved surfaces) is indicated with the referencenumeral 62. As shown in FIG. 1 , the disc 10 may be symmetrical about acentral anterior-posterior plane containing the axis 62. Referring toFIG. 4 , in some embodiments the axis 62 is posteriorly disposed, i.e.is located closer to the posterior limit of the disc than the anteriorlimit thereof.

In use, the disc 10 is surgically implanted between adjacent spinalvertebrae in place of a damaged disc. The adjacent vertebrae areforcibly separated from one another to provide the necessary space forinsertion. The disc is inserted, normally in a posterior direction, intoplace between the vertebrae with the fins 22, 42 of the plates 12, 14entering slots cut in the opposing vertebral surfaces to receive them.After insertion, the vertebrae, facets, adjacent ligaments and softtissues are allowed to move together to hold the disc in place. Theserrated and microfinished surfaces 18, 40 of the plates 12, 14 locateagainst the opposing vertebrae. The serrations 20 and fins 22, 42provide initial stability and fixation for the disc 10. With passage oftime, enhanced by the titanium surface coating, firm connection betweenthe plates and the vertebrae will be achieved as bone tissue grows overthe serrated surface. Bone tissue growth will also take place about thefins 22, 40 and through the transverse holes 23 therein, furtherenhancing the connection which is achieved.

Referring to FIG. 5 , the core 16 may be formed with narrow, angularlyspaced, blind passages 61 which accommodate titanium pins 64. In manyembodiments, the core 16 itself is transparent to X-radiation and so isinvisible in a post-operative X-ray examination. The pins 64 serve asradiographic markers and enable the position of the core 16 to beascertained during such examination.

In the assembled disc 10, the complementary and cooperating sphericalsurfaces of the plates and core allow the plates to slide or articulateover the core through a fairly large range of angles and in alldirections or degrees of freedom, including rotation about the centralaxis 62. FIGS. 1 and 4 show the disc 10 with the plates 12 and 14 andcore 16 aligned vertically with one another on the axis 62. FIG. 2illustrates a situation where maximum anterior flexion of the disc 10has taken place. At this position, the upper rib 56 has entered thehollow 38 of the U-shaped member 30, the lower surface of the rib 56 hasmoved into contact with the upper surface 34 of the flange 28, theflange having moved into the groove 54, and the lower surface 36 of theflange has moved into contact with the upper surface of the ledgeformation 46, as will be seen in the encircled areas 69. Abutmentbetween the various surfaces prevents further anterior flexure. Thedesign also allows for the inner extremity of the flange 28 to abutagainst the base of the groove 54, thereby limiting further relativemovement between the core and plate. A similar configuration is achievedin the event of maximum posterior flexure of the plates 12, 14 over thecore, such as during spinal extension and/or in the event of maximumlateral flexure.

FIG. 3 illustrates how the disc 10 can also allow for translationalmovement of the plates relative to the core. In the illustratedsituation there has been lateral translation of the plates relative tothe core. The limit of lateral translation is reached when the innerextremity of the flange 28 abuts the base of the groove 54 as indicatedby the numeral 70.

The flange 28 and the groove 54 defined between the ribs 56, preventseparation of the core from the plates. In other words, the cooperationof the retaining formations ensures that the core is held captivebetween the plates at all times during flexure of the disc 10.

In an alternative embodiment, the continuous annular flange 28 may bereplaced by a retaining formation comprising a number of flange segmentswhich are spaced apart circumferentially. Such an embodiment couldinclude a single, continuous groove 54 as in the illustrated embodiment.Alternatively, a corresponding number of groove-like recesses spacedapart around the periphery of the core could be used, with each flangesegment opposing one of the recesses. In another embodiment, thecontinuous flange or the plurality of flange segments could be replacedby inwardly directed pegs or pins carried by the upper plate 12. Thisembodiment could include a single, continuous groove 54 or a series ofcircumferentially spaced recesses with each pin or peg opposing arecess.

In yet another embodiment, the retaining formation(s) could be carriedby the lower plate 14 instead of the upper plate, i.e. the plates arereversed. In some embodiments, the upper (or lower) plate is formed withan inwardly facing groove, or circumferentially spaced groove segments,at the edge of its inner, curved surface, and the outer periphery of thecore is formed with an outwardly facing flange or with circumferentiallyspaced flange segments.

Although the foregoing is a complete and accurate description of theinvention, any of a number of modifications, additions or the like maybe made to the various embodiments without departing from the scope ofthe invention. Therefore, nothing described above should be interpretedas limiting the scope of the invention at it is described in the claims.

What is claimed is:
 1. A prosthetic disc for insertion between adjacent vertebrae, the prosthetic disc comprising: upper and lower plates, each plate having an outer surface which engages a vertebra and a curved inner bearing surface; a core having first and second outer surfaces which are each complementary in shape to, mate with and slide over the corresponding bearing surfaces of the upper and lower plates in an implantation configuration when the prosthetic disc is inserted between adjacent vertebrae; and a retaining formation on the lower plate, the retaining formation including at least one inwardly facing groove at an edge of the curved inner bearing surface of the lower plate; wherein an outwardly facing flange defines an outer periphery of the core, the flange configured to engage the retaining formation on the lower plate to hold the core between the upper and lower plates as the core slides over the bearing surface of the lower plate, and wherein the entire core is moveable with respect to the upper and lower plates in the implantation configuration.
 2. The disc of claim 1, wherein the upper and lower plates are metallic and the core comprises a polymer.
 3. The disc of claim 2, wherein the core comprises polyethylene.
 4. The disc of claim 1, wherein the outwardly facing flange comprises a number of spaced flange segments.
 5. The disc of claim 1, wherein at least one of the curved inner bearing surfaces and at least one of the first and second outer surface are cooperating spherical surfaces.
 6. The disc of claim 1, wherein the outer surfaces of the upper and lower plates each comprise an upstanding central fin.
 7. The disc of claim 6, wherein the outer surfaces of the upper and lower plates comprise a rough microfinish.
 8. The disc of claim 7, wherein the outer surfaces of the upper and lower plates are coated with titanium plasma spray.
 9. The disc of claim 1, wherein the core includes convex outer surfaces on both the upper and lower surfaces.
 10. The disc of claim 1, wherein no retaining formation is provided on the upper plate.
 11. The disc of claim 1, wherein no inwardly facing groove is provided on the upper plate.
 12. A method of retaining a core in a prosthetic disc for insertion between adjacent vertebrae, the method comprising: providing upper and lower plates, each plate having an outer surface which engages a vertebra and a curved inner bearing surface; providing a retaining formation on the lower plate, the retaining formation including at least one inwardly facing grove at an edge of the curved inner bearing surface of the lower plate; inserting a core between the upper and lower plates, the core having first and second outer surfaces which are each complementary in shape to, mate with and slide over the corresponding bearing surfaces of the upper and lower plates in an implantation configuration when the prosthetic disc is inserted between adjacent vertebrae, wherein an outwardly facing flange defines an outer periphery of the core; holding the core between the upper and lower plates by causing the at least one outwardly facing flange of the core to engage the retaining formation on the lower plate to hold the core between the upper and lower plates as the core slides over the bearing surface of the lower plate, while allowing the upper and lower plates to move with respect to one another and with respect to the entire core.
 13. The method of claim 12, wherein the upper and lower plates are metallic and the core comprises a polymer.
 14. The method of claim 13, wherein the core comprises polyethylene.
 15. The method of claim 12, wherein the outwardly facing flange comprises a number of spaced flange segments.
 16. The method of claim 12, further comprising providing the outer surfaces of the upper and lower plates each with an upstanding central fin for fixation.
 17. The method of claim 16, wherein the outer surfaces of the upper and lower plates comprise a rough microfinish for further fixation.
 18. The method of claim 16, wherein the outer surfaces of the upper and lower plates are coated with titanium plasma spray for further fixation.
 19. The method of claim 12, wherein no retaining formation is provided on the upper plate. 